It is often desirable commercially and industrially to dissolve a gas in an aqueous body or stream. Some gases which are dissolved in aqueous streams include chlorine, chlorine dioxide, oxygen, ozone and carbon dioxide. Even though some gases dissolve faster, and have a greater solubility, than others it is generally recognized as a difficult control problem to feed a gas into the water at a rate which is acceptable for efficiency and yet will lead to essentially total dissolution of the gas volume feed. The limited contact time in which dissolution can be effected, when it is considered that gas bubbles rise at a rate of about 0.75 to 1 ft/sec when a gas is fed into the bottom of an aqueous pool, is often not enough to prevent undissolved gas from escaping and being lost when the water retaining means is open to the atmosphere or has a top gas collecting space.
One area where gas dissolution is particularly important is in a water treatment plant, especially a city or municipal plant. It is very common in such plants to include a lime treatment step to soften the water by removing calcium and also at times magnesium. Sodium carbonate is also added if the water contains non-carbonate hardness, usually in the form of sulfates of calcium or magnesium. The result of such treatment is that the pH of the water is raised above 10. This leads to unstable water that is supersaturated with calcium carbonate, which will subsequently be converted into an insoluble form and precipitate on filter media and in plant and distribution system piping and equipment. To convert the unstable water to a stable form it has been conventional to dissolve carbon dioxide gas in the water in a recarbonation basin so that the carbonic acid which forms can react with the alkaline materials in the water and lower the pH to about 8 to 9. This leads to stable water having carbonate equilibrium so there is neither carbonate scaling upon further handling and transport of the water where it is to be used, nor is the water corrosive to piping and equipment.
After dissolution of the carbon dioxide in the lime softened water, some retention time is necessary to complete the reaction between dissolved carbon dioxide and the hydroxide and carbonate ions. This time may vary from 10 to as much as 30 minutes, depending on the chemical quality and the temperature of the specific water.
Waters treated to a high pH of around 11 for magnesium removal contain excess lime which is removed as precipitated calcium carbonate as the pH is lowered by carbon dioxide. Further additon of carbon dioxide converts remaining normal dissolved calcium carbonates to bicarbonates to the extent required to produce a stable water. This type of water, especialy if cold, will require a longer reaction time, whereas a warm water softened to a pH of about 10 for calcium removal only has to convert dissolved carbonates to bicarbonates, and the reaction time can be shorter.
The quantity of carbon dioxide required for stabilization depends upon the specific chemical quality and pH of the water involved, and it may range from about 100 to 300 pounds of carbon dioxide per million gallons of water treated. Based on feeding pure carbon dioxide gas with a specific volume of 8.73 cubic feet per pound at 70.degree. F. and 1 atmosphere pressure, it would require a range of 0.606 to 1.82 cubic feet per minute of carbon dioxide gas per 694.4 gallons per minute of water (1 million gallons per day), assuming complete dissolution of the carbon dioxide gas. This is a volume ratio of 0.6 to 1.82 cubic feet of carbon dioxide per 92.83 cubic feet of water.
Water treatment plants which include a carbon dioxide dissolution step generally feed the gas into the bottom of a tank or vessel containing the water to be treated. The tank is usually partially or wholly open at the top, so that undissolved gas which reaches the water surface escapes and is lost. This represents an economic loss. While the loss has been noted for many years there is need for a more economical solution to the problem. Any solution must obviously be achieved with minimum capital investment and low operating cost since the goal is to provide high quality water in high volume at minimum cost. Equipment and methods used must be easy to operate, have high reliability, be inexpensive and be uncomplicated. Such goals are met by the apparatus and methods of dissolving gas in an aqueous pool or stream according to the invention.